Astronomers have used a principle first proposed by Albert Einstein over 100 years ago to map the distribution of dark matter in unprecedented detail. The team’s method revealed the presence of “clumps” of dark matter between galaxies, showing how this mysterious substance is distributed on smaller scales.
Fluctuations in the observed dark matter, identified between a distant quasar (or a bright light source powered by a feeding supermassive black hole) and a galaxy located between that quasar and Earth, could help constrain the matter’s elusive properties.
Dark matter is worrisome to scientists, because although it makes up about 85% of the universe, it is virtually invisible. This is because dark matter either does not interact with any electromagnetic radiation, including visible light, or it interacts very weakly.
This means that the particles that make up dark matter – whatever they are – cannot be atoms made up of electrons, protons and neutrons. These are the baryons that make up the everyday matter that makes up stars, planets, our bodies, and everything we see around us on a daily basis.
It is this mystery that has prompted the intense search for so-called dark matter particles.
Related: We still don’t know what dark matter is, but here’s what it isn’t
Until now, the only way scientists could infer the existence of dark matter was to look at its effect on “normal” matter via gravity. In fact, when astronomers did this, they found that if galaxies weren’t made up mostly of dark matter, their contents would quickly fly out because they were spinning too fast to be held together by the gravity of the visible matter inside them.
Not only are galaxies surrounded by halos of dark matter thought to prevent such a catastrophe, but some dark matter models also suggest that there must be clumps of dark matter inside galaxies as well as filling the space between them.
A team of researchers from Japan, led by Kalki Taro Inoue of Kindai University, set out to use the Atacama Large Millimeter/Submillimeter Array (ALMA) to better understand the distribution of dark matter around a massive, distant galaxy and find clumps of mysterious matter. in intergalactic space.
To do this, they observed the light from a quasar called MG J0414+0534, located 11 billion light-years from Earth, using an effect called gravitational lensing.
Putting dark matter under a cosmic magnifying glass
Gravitational lensing is a concept that first emerged from Einstein’s theory of gravity, general relativity, which was published in 1915. This concept differed radically from Newton’s theory of gravity because it reimagined the fabric of space and time – united as four-dimensional spacetime – as shown in the image. A dynamic element in the universe, not just a static stage on which cosmic events take place.
Einstein envisioned that objects with mass cause a curvature, or “warp,” of the fabric of space-time. The greater the mass, the greater the curvature in space-time. This can be pictured as a simple two-dimensional case of objects being placed on a stretched rubber sheet. A bowling ball will poke a larger hole in paper than a tennis ball, just as a galaxy creates a larger curve in space-time than a star.
Moreover, something truly remarkable happens when a massive object of great mass comes between Earth and a distant source of light such as another galaxy, a star, or in this case, a quasar. Normally, light travels in a straight line to Earth, but when it goes through this curvature of space, its path also becomes curved. Masses closer to our planet that cause such curvature lead to more extreme deviations.
This means that light from a single source can take different paths around a massive object and thus reach the telescope at different times. This can cause a single object to be brightened and magnified in the image or even appear in multiple places in the same image.
The intervening object is therefore referred to as a gravitational lens.
Gravitational lensing can help scientists see objects that are usually too far away and dim to observe. For example, the James Webb Space Telescope (JWST) used gravitational lensing to great effect to see galaxies in the early universe.
But in addition to helping scientists study the topic of gravitational lensing, the effect can also be used to map the distribution of matter in a galaxy that acts as a cosmic lens in the first place. This includes mapping dark matter.
Thus, astronomers were able to map the distribution of visible matter, and then infer the distribution of dark matter in lenticular galaxies.
Inoue and his team did just that for dark matter in the galaxy, focusing on the distant quasar subject, causing MG J0414+0534 to appear four times in a single ALMA observation. This allowed the researchers to capture images of the galaxy at higher resolution than ever before, mapping its dark matter on a scale of up to 30,000 light-years.
They were also able to advance the application of gravitational lensing.
Thanks to ALMA’s high resolution, astronomers have been able to map the distribution of dark matter clumps that lie between galaxies and along the line-of-sight of a quasar 11 billion light-years away. The findings presented by the researchers help confirm the so-called “cold dark matter” (CDM) model of the universe, which suggests that dark matter is composed of slow-moving particles.
This is because the CDM model predicts clumps of dark matter both inside and outside galaxies, distributed across intergalactic space.
The team’s research was published Thursday (September 7) in The Astrophysical Journal.
